Little by little, lightning sensors in space are revealing the inner workings of severe storms. Scientists hope to use the technique to improve forecasts of deadly weather.

by Annie Strickler.

For most people watching a thunderstorm roll in, the bolts of lightning are just streaks of white light setting the night on fire, ushering in the crackle and rumble of thunder. But these brilliant flashes of lightning - electrical discharges between the positive and negative regions of a thunderstorm - also illuminate the workings of the atmosphere, providing information about storms that can improve emergency response efforts, saving money and lives.

To harness this information, NASA utilizes a fleet of ground-based, airborne and space-based sensors to detect lightning and characterize the electrical behavior of storms - all in the pursuit of advances in climatology and "nowcasting."

More accurate and timely forecasting, or "nowcasting," would help people gauge evacuation measures, help aviation officials map routes and plan refueling operations, provide better storm tracking, prevent systems disruptions and minimize hazards to NASA's spacecraft launches. Another potential benefit is providing algorithms for forecasting the likelihood of forest fires.

"Incremental gains ... really translate into cost and life benefits," says Dr. Dennis Boccippio, an atmospheric scientist at NASA's Marshall Space Flight Center in Huntsville, Ala.

NASA The awesome power of nature.

Current ground observations require about five minutes to scan a storm and make a report on its characteristics. But, says Boccippio, "there's a lot of evolution that can go on in a storm within that five minutes." And infrared satellite observations - from which we get those famous hurricane tracking reports that dominate television weather coverage in the late summer and early fall - can take 20 to 30 minutes to scan the "disk" of the Earth that's visible to the satellite. As Boccippio says, and as many storm victims probably agree, "You pay the price for that." Updating lightning flash rates at one-minute intervals would benefit "nowcasting" efforts, according to Boccippio.

To this end, a collective goal of researchers under the lead of Dr. Hugh Christian at NASA and the Global Hydrology and Climate Center (GHCC) in Huntsville, Ala., is to place a lightning sensor in geosynchronous orbit so that scientists can monitor storms over their entire life cycles. This sensor, called the Lightning Mapper Sensor (LMS), "would essentially rotate with the Earth," giving it a constant view of storms, Boccippio says.

"The end applications goal," says Boccippio, "is to improve real-time forecasting. ... It is the rapid updates that forecasters are excited about."

Measuring lightning from space is relatively simple and inexpensive. The satellites have some fancy optics, but Boccippio says they are "essentially glorified digital video cameras."

One of their unique characteristics is the ability to detect lightning during the day when the human eye cannot sense it. Furthermore, because of lightning's "impulsive, event-based" nature, the data sets are relatively small in size. The promise is that the data will be easier to deal with and to distribute to users.

The Lightning Team has already successfully developed and flown two optical lightning detectors. The first was the Optical Transient Detector (OTD). This "large-scale climatology instrument" collected a five-year record of lightning observations between April 1995 and April 2000.

The Tropical Rainfall Measuring Mission (TRMM) was launched in November 1997 and has been providing high-resolution images and rainfall measurements for the tropics between roughly 35 degrees north and south latitudes. TRMM carries five unique sensory instruments, including the Lightning Imaging Sensor (LIS) that enables scientists to study the distribution and variability of global lightning.

The Tropical Rainfall Measuring Mission (TRMM) was launched in November 1997 and has been providing high-resolution images and rainfall measurements for the tropics between roughly 35 degrees north and south latitudes. TRMM carries five unique sensory instruments, including the Lightning Imaging Sensor (LIS) that enables scientists to study the distribution and variability of global lightning.

Boccippio and Goodman work at the National Space Science and Technology Center in Huntsville, Ala., as part of the Lightning Team. Cummins - from Global Atmospherics, Inc. - represents the commercial side of the investigation. The company operates the National Lightning Detection Network (NLDN), a network of about 130 time-of-arrival and magnetic direction finders covering the United States.

The Monthly Weather Review paper is based on four years of observations from OTD and NLDN. The authors feel that while significant research has been dedicated to variations in lightning flash rates, there has been a dearth of research on the relative proportions of intracloud and cloud-to-ground lightning.

NASA At least 130 time-of-arrival and magnetic direction finders, as shown in the image on the left, are positioned all over the U.S. to locate lightning strikes. Each sensor can detect the direction of a strike that's 400+ kilometers away. The strike's location is determined by triangulation.

The ratio of intracloud to cloud-to-ground lightning can help scientists detect and interpret anomalies in severe storms. Intracloud lightning is the most common and appears as channels of light emanating from a central point. Cloud-to-ground lightning is less common but more dangerous. The former type of lightning is weaker and harder to measure over long distances, while the latter is easier to measure from the ground. Global-scale measurement of both types is easier from space.

The ratio between the two types of lightning varies from storm to storm. In the past, it has been difficult to get a baseline for that ratio. As evidenced by this paper, merging satellite and ground measurements establishes an average that enables scientists to identify an anomalous ratio, or "one that is much higher than the garden variety," as Boccippio put it.

The scientists found that, in the Midwest and the Great Plains, storms were more severe on average, and the ratio leaned toward intracloud lightning. In the South, where storms tend to be less severe than they are in the Midwest, there was a lower intracloud to cloud-to-ground lightning ratio. The long-term average in the Southeast turned out to be 3:1, while in the Midwest it was 10:1 on average and much higher during severe storms. One of their key scientific findings, Boccippio says, was determining "an average for the U.S….and finding that there was significant variability by region."

While this may not constitute a shattering of paradigms, their work is a step toward NASA's goal of advancing climatology and real-time storm "nowcasting." However, Boccippio notes that there are still "basic unanswered questions [that] need to be tackled."